I've never seen anyone guarantee 100% uptime.
but I suspect that it is possibly.
95% uptime means that you are agreeing to the risk of technical unavailability for 18.25 days.
100% uptime means that you're agreeing to not loose connectivity.
the thing that you have to remember is that maintenance time is not included in the figures, so, with a 100% availability guarantee they can email ou a week in advance saying that they are performing maintenance and that your system will be unavailable for say an hour.
even though you've paid for 100% uptime, you'll have usually agreed that this excludes agreed uptime.
95% uptime has the same problems.
in the worst case you could have 18.25 days of downtime in a year, as well as maintenance downtime...
just decide how critical your site is to yu really I guess.